Many things go into making rivalries and over time they may change. In thr early days of football P-Y seemed a natural as they were the dominant powers of the first half century. Harvard then attempted to strengthen its program to the level of Y-P and supplant P. Not sure how much Y cared as it had two teams viewing it as the major rival. When I was a kid in the 50s growing up in Princeton, Harvard was viewed as an appetizer to our real rival, Yale. But, Dartmouth was always strong and was our final game plus the result often determined the IL champion. More recently, Penn has risen to dominance in the IL and seems an appropriate rival to P, especially since some of its (Penn’s) fans believe that it does not receive its share of Ivy notoriety and focus that resentment on Princeton. As far as Tiger fans are concerned, Yale and whoever is the current league power share the role of principal rival. Personally, I enjoy beating Harvard as much as anyone as the IL in general closes the celebrity gap between H and the rest of the league (perhaps a mirror of the way some Penn fans view Princeton).